Closed content distribution systems include end-to-end systems, including publishing servers, content distribution servers and playback devices, where the content that is playable on playback devices can be completely controlled through appropriate security techniques, and those security techniques make it relatively difficult for any unauthorized third party to distribute content that would be playable on the playback devices.
In known computing systems, content can be delivered in an encrypted form, so that unauthorized recipients are unable to use that content without obtaining a decryption key. In its encrypted form, the content can be delivered either directly from a content server, or more indirectly using one or more intermediate servers such as caching devices. The key can be delivered separately from a key server to authorized recipients only. The key is in general is much smaller than the content, with the effect that the key can be encrypted separately for each authorized recipient without involving significant amounts of communication or computation resources. One effect of delivering the key to only authorized recipients is that only those authorized recipients are able to use the content. In addition to a key, a secure hash or other confirming signature can be delivered separately from the content, such as from the key server, again only to authorized recipients, with the effect that those authorized recipients can verify the authenticity of the content they receive and decrypt.
A first problem in the known art is that if the original content encryption key is compromised (either by being broken computationally or by being disclosed in an unauthorized manner), it becomes possible for a third party to use that encryption key to distribute unauthorized content to playback devices, such as by encrypting the unauthorized content using the compromised encryption key. This is sometimes called “content spoofing.” This problem can be ameliorated by including a secure hash along with the encryption key. In such cases the playback device's security software would load and authenticate the entire content before playback, so that unauthorized content encrypted with the compromised key would be detected and the playback device's security software would refuse to play that unauthorized content.
A second problem in the known art occurs if the content is sufficiently large that it must be stored on an insecure storage device (such as for example an external mass storage device) and re-read from that insecure storage device dynamically during playback. In this case, even authenticating the entire content before playback will only provide relatively limited protection against content spoofing, since a sophisticated attacker would be able to replace the content seen by the playback device after its authentication step was completed.
A first possible solution to this problem is to assign separate signatures to each small chunk of content that is loaded from the insecure storage device. While this possible solution achieves the general goal of preventing content spoofing for each chunk individually, it has the initial drawback that in practical systems this technique might require significant computational and communication resources if the loading occurs in large numbers of pieces.
A second possible solution to this problem is for the content server to individually encrypt each copy of the content for the specific authorized recipient of that copy, with the effect that an unauthorized distributor who wishes to serve content to the population of players would have to obtain the specific key for each player to be distributed to. While this possible solution achieves the general goal of preventing content spoofing individual authorized recipients, it is has the initial drawbacks that (1) encrypting each copy of the content for each specific authorized recipient involves significant computation resources at the content server, and (2) the individually encrypted copies of the content for each specific authorized recipient cannot readily be cached by intermediate servers, thus involving significantly greater amounts of communication resources when distributing the content to authorized recipients.
The foregoing examples of the related art and limitations related therewith are intended to be illustrative and not exclusive. Other limitations of the related art will become apparent upon a reading of the specification and a study of the drawings.